Pkd2 haploinsufficiency alters intracellular calcium regulation in vascular smooth muscle cells

Autosomal-dominant polycystic kidney disease is a multiorgan disease and its vascular manifestations are common and life-threatening. Despite this, little is known about their pathogenesis. Somatic mutations to the normal PKD allele in cystic epithelia and cyst development associated with the unstable Pkd2^WS25 allele suggest a two-hit model of cystogenesis. However, it is unclear if this model can account for the cardiovascular pathology or if haploinsufficiency alone is disease-associated. In the present study, we found a decreased polycystin-2 (PC2, protein encoded by Pkd2 gene) expression in Pkd2^+/- vessels, roughly half the wild-type level, and an enhanced level of intracranial vascular abnormalities in Pkd2^+/- mice when induced to develop hypertension. Consistent with these observations, freshly dissociated Pkd2^+/- vascular smooth muscle cells have significantly altered intracellular Ca²⁺ homeostasis. The resting [Ca²⁺], is 17.1% lower in Pkd2^+/- compared with wild-type cells (P=0.0003) and the total sarcoplasmic reticulum Ca²⁺ store (emptied by caffeine plus thapsigargin) is decreased (P<0.0001). The store operated Ca²⁺ (SOC) channel activity is also decreased in Pkd2^+/- cells (P=0.008). These results indicate that inactivation of just one Pkd2 allele is sufficient to significantly alter intracellular Ca²⁺ homeostasis, and that PC2 is necessary to maintain normal SOC activity and the SR Ca²⁺ store in VSMCs. Based on these findings, and the fact that [Ca²⁺]_i signaling is essential to the regulation of contraction, production and secretion of extracellular matrix, cellular proliferation and apoptosis, we propose that the abnormal intracellular Ca²⁺ regulation associated with Pkd2 haploinsufficiency is directly related to the vascular phenotype.